1. Field of the Invention
The present invention relates to a discharge circuit in a DC power supply, for example, to a technology suitable for a discharge circuit for an output capacitor in a regulating semiconductor integrated circuit of a series regulator.
2. Description of Related Art
A series regulator is a type of DC power supply designed to feed a predetermined output voltage by decreasing an input voltage through voltage regulation with a regulator transistor in response to a required output voltage. Among such series regulators employed in electric power sources, a series regulator employed in a particularly noise-sensitive system includes a bipolar transistor as a circuit component and a smoothing capacitor connected to an output terminal, where the smoothing capacitor has larger capacitance compared to one used in a comparatively noise-insensitive system. Additionally, a series regulator with an output capacitor of a large capacitance may include a discharge circuit as shown in FIG. 5 in order to achieve a rapid fall of an output voltage when a power source is turned off.
A discharge circuit in a series regulator shown in FIG. 5 is composed of a discharge transistor Q1 connected between an output terminal OUT and a grounding point GND, voltage dividing resistors R6, R7 serially imposed between OUT and GND, and a transistor Q2 connected in series to the resistors. When an external control signal ON/OFF for switching on/off a power source takes a voltage corresponding to an the OFF state of the power source, the transistor Q2 turns off, and the transistor Q1 turns on in turn. The transistor Q1 then acts to rapidly lower the output voltage Vout by withdrawing an electric charge from the capacitor Co connected to the output terminal OUT. An invention regarding a series regulator including such a discharge circuit is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2000-066742.
A regulator including a discharge circuit shown in FIG. 5 is configured to supply a base current of the discharge transistor Q1 from the capacitor connected to the output terminal OUT. Accordingly, if the output voltage Vout goes down to 0.7 V, which corresponds to a voltage between a base and an emitter of Q1, Q1 is turned off and a discharge operation is then stopped. Thus the output voltage Vout cannot fall down to 0 V.
In order to address the problems described above, as illustrated in FIG. 6, a discharge circuit may be configured to turn on/off a discharge transistor Q1 with a voltage applied to an input (i.e., a voltage from a biasing circuit) Unfortunately a regulator including such a discharge circuit suffers a problem of wasteful current consumption since a constant current continues to flow through the base of the discharge transistor Q1 while the regulator is turned off.
An object of the present invention is to provide a semiconductor integrated circuit for a regulator which can decrease a wasteful current consumption while the regulator is being turned off, and enable an output voltage to drop rapidly to a level around ground voltage (0 V).